Laser Processing Method and Semiconductor Device

ABSTRACT

A laser processing method which can reliably form a modified region within an object to be processed along a desirable part in a line to cut is provided. 
     This laser processing method irradiates a substrate  4  with laser light L while locating a light-converging point P within the substrate  4 , so as to form a quality modified region  71  to become a starting point region for cutting within the substrate  4  along a line to cut  5 . Here, the laser light L is oscillated pulsewise along a desirable part RP in the line to cut  5 , and continuously in a predetermined part RC in the line to cut  5 . Consequently, the quality modified region  71  is formed within the substrate  4  along the desirable part RP in the line to cut  5 , whereas no quality modified region  71  is formed within the substrate  4  along the predetermined part RC in the line to cut  5.

TECHNICAL FIELD

The present invention relates to a laser processing method used forcutting an object to be processed, and a semiconductor apparatusmanufactured by using the same.

BACKGROUND ART

Known is a laser processing method which, when cutting an object to beprocessed upon irradiation with laser light, irradiates the object withthe laser light while switching between continuous oscillation and pulseoscillation (see, for example, Patent Document 1). This laser processingmethod oscillates the laser light continuously in linear parts in linesto cut, and pulsewise in curved or corner parts in the lines to cut.

-   Patent Document 1: Japanese Patent Application Laid-Open No. SHO    59-212185

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

Known on the other hand is a laser processing method which irradiates aplanar object to be processed with laser light while locating alight-converging point within the object, so as to form a modifiedregion to become a starting point region for cutting within the objectalong a line to cut the object. Such a laser processing method has beendesired to reliably form the modified region within a desirable part inthe line to cut.

In view of such circumstances, it is an object of the present inventionto provide a laser processing method which can reliably form a modifiedregion within an object to be processed along a desirable part in a lineto cut, and a semiconductor apparatus manufactured by using the same.

Means for Solving Problem

For solving the above-mentioned problem, in one aspect, the presentinvention provides a laser processing method of irradiating a planarobject to be processed with laser light while locating alight-converging point within the object so as to form a modified regionto become a starting point region for cutting within the object along aline to cut the object, the method selectively switching betweencontinuous oscillation and pulse oscillation when irradiating the objectwith the laser light.

Oscillating the laser light pulsewise can form a modified region withinthe object more reliably than with the case where the laser light iscontinuously oscillated. Therefore, oscillating the laser lightpulsewise along a desirable part in the line to cut and continuously inthe part other than the desirable part can reliably form a modifiedregion within the object along the desirable part. Especially when aQ-switched laser is used, the Q-switch is under ON-control by control ofRF-output such that the pulsed oscillation and the continuousoscillation are switched therebetween, so an application state of LDlight for exciting to a solid state laser crystal does not changebasically. Therefore, the pulsed oscillation and the continuousoscillation can be speedily switched therebetween, so processing can beperformed with the stable laser light and speed for processing can begathered. When the continuous oscillation output and the pulsedoscillation output are mixed during the continuous oscillation by a typeof a laser oscillator, average output of pulsed output is low, so theenergy does not go over a threshold for processing, and the modifiedregion is not formed within the object in the part other than adesirable part. In this case, the pulsed oscillation and the continuousoscillation can be speedily switched therebetween and heat stability canprogress when the pulsed oscillation is switched, so processing can beperformed with the stable laser light and speed for processing can begathered. The continuous oscillation in the present invention includesthis case.

Preferably, the object is a substrate having a front face formed with alaminate part, whereas the modified region is formed within thesubstrate. In this case, oscillating the laser light pulsewise along adesirable part in the line to cut and continuously in the part otherthan the desirable part can reliably form a modified region within thesubstrate along the desirable part.

Preferably, the modified region is formed at a position where a distancebetween the front face and an end part on the front face side of themodified region is 5 μm to 20 μm. Preferably, the modified region isformed at a position where the distance between the front face and anend part on the rear face side of the modified region is [5+(substratethickness)×0.1] μm to [20+(substrate thickness)×0.1] μm. Here, the“distance” refers to the distance along the thickness direction of thesubstrate unless otherwise specified.

For example, when an expandable film such as an expandable tape isattached to the rear face of the substrate and expanded in the casementioned above, the substrate and laminated part are cut along the lineto cut. When the modified region is formed at any of the positionsmentioned above, the laminate part can be cut with a high precision.

When the laminate part includes a metal film or insulating film along apredetermined part in the line to cut, it will be preferred if the laserlight is continuously oscillated in the predetermined part. Damagesimparted to the laminate part can be made smaller in this case than inthe case where the laser light is oscillated pulsewise along thepredetermined part. This can improve the accuracy in cutting thelaminate part in the predetermined part in the line to cut when cuttingthe substrate and laminate part.

Preferably, the laser light is oscillated pulsewise in a part wherelines to cut intersect. This reliably forms the modified region withinthe object along the part where the lines to cut intersect. As aconsequence, the accuracy in cutting the object can improve in the partwhere the lines to cut intersect.

Preferably, the object is cut along the line to cut after forming themodified region. This can cut the object with a high precision along theline to cut.

In another aspect, the present invention provides a semiconductorapparatus manufactured by using the laser processing method mentionedabove. This semiconductor apparatus has a cut section cut with a highprecision.

Effect of the Invention

The present invention can reliably form a modified region within anobject to be processed along a desirable part in a line to cut.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an object to be processed during laserprocessing by the laser processing method in accordance with anembodiment;

FIG. 2 is a sectional view of the object taken along the line II-II ofFIG. 1;

FIG. 3 is a plan view of the object after the laser processing by thelaser processing method in accordance with the embodiment;

FIG. 4 is a sectional view of the object taken along the line IV-IV ofFIG. 3;

FIG. 5 is a sectional view of the object taken along the line V-V ofFIG. 3;

FIG. 6 is a plan view of the object cut by the laser processing methodin accordance with the embodiment;

FIG. 7 is a graph showing relationships between the field intensity andcrack spot size in the laser processing method in accordance with theembodiment;

FIG. 8 is a sectional view of the object in a first step of the laserprocessing method in accordance with the embodiment;

FIG. 9 is a sectional view of the object in a second step of the laserprocessing method in accordance with the embodiment;

FIG. 10 is a sectional view of the object in a third step of the laserprocessing method in accordance with the embodiment;

FIG. 11 is a sectional view of the object in a fourth step of the laserprocessing method in accordance with the embodiment;

FIG. 12 is a view showing a photograph of a cross section of a part of asilicon wafer cut by the laser processing method in accordance with theembodiment;

FIG. 13 is a graph showing relationships between the laser lightwavelength and the transmittance within the silicon substrate in thelaser processing method in accordance with the embodiment;

FIG. 14 is a plan view of the object in the laser processing method ofthe embodiment;

FIG. 15 is a partial sectional view of the object taken along the lineXV-XV of FIG. 14;

FIG. 16 is a view for explaining the laser processing method of theembodiment, in which (a) and (b) show respective states where aprotective tape is attached to the object and where the object isirradiated with laser light;

FIG. 17 is a view for explaining the laser processing method of theembodiment, in which (a) and (b) show respective states where anexpandable tape is attached to the object and where the protective tapeis irradiated with UV rays;

FIG. 18 is a view for explaining the laser processing method of theembodiment, in which (a) and (b) show respective states where theprotective tape is peeled off from the object and where the expandabletape is expanded;

FIG. 19 is a plan view showing a part of the object having a modifiedregion formed by the laser processing method of the embodiment;

FIG. 20 is a partial sectional view of the object taken along the lineXX-XX of FIG. 19;

FIG. 21 is a plan view of the object in an example of the laserprocessing method in accordance with the embodiment, in which (a) and(b) show respective states after the modified region is formed withinthe object and after the object is cut;

FIG. 22 is a view showing a photograph of a cut section of the objectcut by the example of the laser processing method in accordance with theembodiment;

FIG. 23 is a plan view of the object in an example of laser processingmethod, in which (a) and (b) show respective states after the modifiedregion is formed within the object and after the object is cut; and

FIG. 24 is a view showing a photograph of a cut section of the object inthe example of laser processing method.

EXPLANATIONS OF NUMERALS OR LETTERS

1 . . . object to be processed; 3 . . . front face; 4 . . . substrate; 4a . . . cross section (side face); 5 . . . line to cut; 7 . . . modifiedregion; 8 . . . cutting start region; 13 . . . molten processed region;16 . . . laminate part; 25 . . . semiconductor chip (semiconductorapparatus); 71 . . . quality modified region; 71 a . . . front-side endpart; 71 b . . . rear-side end part; CP . . . part where lines to cutintersect; L . . . laser light; M . . . metal film; P . . .light-converging point; RC . . . predetermined part in lines to cut.

BEST MODES FOR CARRYING OUT THE INVENTION

In the following, a preferred embodiment of the present invention willbe explained in detail with reference to the drawings. The laserprocessing method in accordance with this embodiment utilizes aphenomenon of multiphoton absorption in order to form a modified regionwithin an object to be processed. Therefore, a laser processing methodfor forming a modified region by the multiphoton absorption will beexplained first.

A material becomes optically transparent when its absorption bandgapE_(G) is greater than photon energy hν. Hence, a condition under whichabsorption occurs in the material is hν>E_(G). However, even whenoptically transparent, the material generates absorption under acondition of nhν>E_(G) (where n=2, 3, 4, . . . ) if the intensity oflaser light becomes very high. This phenomenon is known as multiphotonabsorption. In the case of pulsed waves, the intensity of laser light isdetermined by the peak power density (W/cm²) of laser light at alight-converging point. The multiphoton absorption occurs under acondition where the peak power density is 1×10⁸ (W/cm²) or greater, forexample. The peak power density is determined by (energy of laser lightat the light-converging point per pulse)/(beam spot cross-sectional areaof laser light×pulse width). In the case of continuous waves, theintensity of laser light is determined by the field intensity (W/cm²) oflaser light at the light-converging point.

The principle of the laser processing method in accordance with theembodiment using such multiphoton absorption will be explained withreference to FIGS. 1 to 6. As shown in FIG. 1, on a front face 3 of awafer-like (planar) object to be processed 1, a line to cut 5 forcutting the object 1 exists. The line to cut 5 is a virtual lineextending straight. As shown in FIG. 2, the laser processing method inaccordance with this embodiment irradiates the object 1 with laser lightL while locating a light-converging point P within the object 1 under acondition generating multiphoton absorption, so as to form a modifiedregion 7. The light-converging point P is a position at which the laserlight L is converged. The line to cut 5 may be curved instead of beingstraight, and may be a line actually drawn on the object 1 without beingrestricted to virtual lines.

The laser light L is relatively moved along the line to cut 5 (i.e., inthe direction of arrow A in FIG. 1), so as to shift the light-convergingpoint P along the line to cut 5. Consequently, as shown in FIGS. 3 to 5,the modified region 7 is formed within the object 1 along the line tocut 5, whereas a cutting start region 8 is formed by the modified region7. Here, the cutting start region 8 refers to a region which becomes astart point for cutting (fracturing) at the time when the object 1 iscut. The cutting start region 8 may be made by the modified region 7formed continuously or modified regions 7 formed intermittently.

In the laser processing method in accordance with this embodiment, themodified region 7 is not formed by the heat generated from the object 1absorbing the laser light L. The laser light L is transmitted throughthe object 1, so as to generate multiphoton absorption therewithin,thereby forming the modified region 7. Therefore, the front face 3 ofthe object 1 hardly absorbs the laser light L and does not melt.

When the cutting start region 8 is formed within the object 1, fracturesare likely to start from the cutting start region 8, whereby the object1 can be cut with a relatively small force as shown in FIG. 6.Therefore, the object 1 can be cut with a high precision withoutgenerating unnecessary fractures greatly deviating from the line to cut5 on the front face 3 of the object 1.

There seem to be the following two ways of cutting the object 1 from thecutting start region 8 acting as the start point. The first case iswhere an artificial force is applied to the object 1 after forming thecutting start region 8, so that the object 1 fractures from the cuttingstart region 8, whereby the object 1 is cut. This is the cutting in thecase where the object 1 has a large thickness, for example. Applying anartificial force refers to exerting a bending stress or shear stress tothe object 1 along the cutting start region 8, or generating a thermalstress by applying a temperature difference to the object 1, forexample. The other case is where the forming of the cutting start region8 causes the object 1 to fracture naturally in its cross-sectionaldirection (thickness direction) from the cutting start region 8 actingas a start point, thereby cutting the object 1. This becomes possible ifthe cutting start region is formed by one row of modified region 7 whenthe object 1 has a small thickness, or if the cutting start region 8 isformed by a plurality of rows of modified regions 7 in the thicknessdirection when the object 1 has a large thickness. Even in thisnaturally fracturing case, fractures do not extend onto the front face 3at a portion corresponding to an area not formed with the cutting startregion 8, so that only the portion corresponding to the area formed withthe cutting start region 8 can be cleaved, whereby cleavage can becontrolled well. Such a cleaving method with a favorable controllabilityis quite effective, since the object 1 such as silicon wafer hasrecently been apt to decrease its thickness.

The modified region formed by multiphoton absorption in the laserprocessing method in accordance with this embodiment encompasses thefollowing cases (1) to (3):

(1) Case where the modified region is a crack region including one crackor a plurality of cracks

An object to be processed (e.g., glass or a piezoelectric material madeof LiTaO₃) is irradiated with laser light while locating alight-converging point therewithin under a condition with a fieldintensity of at least 1×10⁸ (W/cm²) at the light-converging point and apulse width of 1 μs or less. This magnitude of pulse width is acondition under which a crack region can be formed only within theobject while generating multiphoton absorption without causingunnecessary damages on the front face of the object. This generates aphenomenon of optical damage by multiphoton absorption within theobject. This optical damage induces a thermal distortion within theobject, thereby forming a crack region therewithin. The upper limit offield intensity is 1×10¹² (W/cm²), for example. The pulse width ispreferably 1 ns to 200 ns, for example. The forming of a crack region bymultiphoton absorption is disclosed, for example, in “Internal Markingof Glass Substrate with Solid-state Laser”, Proceedings of the 45thLaser Materials Processing Conference (December, 1998), pp. 23-28.

The inventors determined the relationship between field intensity andcrack size by an experiment. The following are conditions of theexperiment.

(A) Object to be processed: Pyrex (registered trademark) glass (with athickness of 700 μm)

(B) Laser

light source: semiconductor laser pumping Nd:YAG laser

wavelength: 1064 nm

laser light spot cross-sectional area: 3.14×10⁻⁸ cm²

oscillation mode: Q-switched pulse

repetition frequency: 100 kHz

pulse width: 30 ns

output: output<1 mJ/pulse

laser light quality: TEM₀₀

polarizing property: linear polarization

(C) Condenser lens

transmittance at a laser light wavelength: 60%

(D) Moving rate of the mounting table mounting the object: 100 mm/sec

The laser light quality of ILK° means that the light-convergingcharacteristic is so high that convergence to about the wavelength oflaser light is possible.

FIG. 7 is a graph showing the results of the above-mentioned experiment.The abscissa indicates the peak power density. Since the laser light ispulsed laser light, the field intensity is represented by the peak powerdensity. The ordinate indicates the size of a crack part (crack spot)formed within the object by one pulse of laser light. Crack spots gatherto yield a crack region. The crack spot size is the size of a partyielding the maximum length among forms of crack spots. Data representedby black circles in the graph refer to a case where the condenser lens(C) has a magnification of ×100 and a numerical aperture (NA) of 0.80.On the other hand, data represented by whitened circles in the graphrefer to a case where the condenser lens (C) has a magnification of ×50and a numerical aperture (NA) of 0.55. Crack spots are seen to occurwithin the object from when the peak power density is about 10¹¹ (W/cm²)and become greater as the peak power density increases.

A mechanism by which the objet to be processed is cut by forming a crackregion will now be explained with reference to FIGS. 8 to 11. As shownin FIG. 8, the object 1 is irradiated with laser light L while thelight-converging point P is located within the object 1 under acondition where multiphoton absorption occurs, so as to form a crackregion 9 therewithin along a line to cut. The crack region 9 is a regioncontaining one crack or a plurality of cracks. Thus formed crack region9 yields a cutting start region. A crack further grows from the crackregion 9 acting as a start point (i.e., from the cutting start regionacting as a start point) as shown in FIG. 9, and reaches the front face3 and rear face 21 of the object 1 as shown in FIG. 10, whereby theobject 1 fractures and is consequently cut as shown in FIG. 11. Thecrack reaching the front face 3 and rear face 21 of the object 1 maygrow naturally or as a force is applied to the object 1.

(2) Case where the Modified Region is a Molten Processed Region

An object to be processed (e.g., semiconductor material such as silicon)is irradiated with laser light while locating a light-converging pointwithin the object under a condition with a field intensity of at least1×10⁸ (W/cm²) at the light-converging point and a pulse width of 1 μs orless. As a consequence, the inside of the object is locally heated bymultiphoton absorption. This heating forms a molten processed regionwithin the object. The molten processed region encompasses regions oncemolten and then re-solidified, regions just in a molten state, andregions in the process of being re-solidified from the molten state, andcan also be referred to as a region whose phase has changed or a regionwhose crystal structure has changed. The molten processed region mayalso be referred to as a region in which a certain structure has changedto another structure among monocrystal, amorphous, and polycrystalstructures. For example, it means a region having changed from themonocrystal structure to the amorphous structure, a region havingchanged from the monocrystal structure to the polycrystal structure, ora region having changed from the monocrystal structure to a structurecontaining amorphous and polycrystal structures. When the object to beprocessed is of a silicon monocrystal structure, the molten processedregion is an amorphous silicon structure, for example. The upper limitof field intensity is 1×10¹² (W/cm²), for example. The pulse width ispreferably 1 ns to 200 ns, for example.

By an experiment, the inventors verified that a molten processed regionwas formed within a silicon wafer. The following are conditions of theexperiment.

(A) Object to be processed: silicon wafer (with a thickness of 350 μmand an outer diameter of 4 inches)

(B) Laser

light source: semiconductor laser pumping Nd:YAG laser

wavelength: 1064 nm

laser light spot cross-sectional area: 3.14×10⁻⁸ cm²

oscillation mode: Q-switched pulse

repetition frequency: 100 kHz

pulse width: 30 ns

output: 20 μJ/pulse

laser light quality: TEM₀₀

polarizing property: linear polarization

(C) Condenser lens

magnification: ×50

N.A.: 0.55

transmittance at a laser light wavelength: 60%

(D) Moving rate of the mounting table mounting the object: 100 mm/sec

FIG. 12 is a view showing a photograph of a cross section of a part of asilicon wafer cut by laser processing under the conditions mentionedabove. A molten processed region 13 is formed within the silicon wafer11. The molten processed region 13 formed under the above-mentionedconditions has a size of about 100 μm in the thickness direction.

The fact that the molten processed region 13 is formed by multiphotonabsorption will now be explained. FIG. 13 is a graph showingrelationships between the laser light wavelength and the transmittancewithin the silicon substrate. Here, the respective reflected componentson the front and rear sides of the silicon substrate are eliminated, soas to show the internal transmittance alone. The respectiverelationships are shown in the cases where the thickness t of thesilicon substrate is 50 μm, 100 μm, 200 μm, 500 μm, and 1000 μm.

For example, at the Nd:YAG laser wavelength of 1064 nm, the laser lightappears to be transmitted through the silicon substrate by at least 80%when the silicon substrate has a thickness of 500 μm or less. Since thesilicon wafer 11 shown in FIG. 12 has a thickness of 350 μm, the moltenprocessed region 13 caused by multiphoton absorption is formed near thecenter of the silicon wafer 11, i.e., at a part distanced from the frontface by 175 μm. The transmittance in this case is 90% or more withreference to a silicon wafer having a thickness of 200 μm, whereby thelaser light is absorbed only slightly within the silicon wafer 11 but issubstantially transmitted therethrough. This means that the moltenprocessed region 13 is formed within the silicon wafer 11 not by laserlight absorption within the silicon wafer 11 (i.e., not by usual heatingwith the laser light) but by multiphoton absorption. The forming of amolten processed region by multiphoton absorption is disclosed, forexample, in “Silicon Processing Characteristic Evaluation by PicosecondPulse Laser”, Preprints of the National Meetings of Japan WeldingSociety, Vol. 66 (April, 2000), pp. 72-73.

A fracture is generated in a silicon wafer from a cutting start regionformed by a molten processed region, acting as a start point, toward across section, and reaches the front and rear faces of the siliconwafer, whereby the silicon wafer is cut. The fracture reaching the frontand rear faces of the silicon wafer may grow naturally or as a force isapplied to the silicon wafer. The fracture naturally growing from thecutting start region to the front and rear faces of the silicon waferencompasses a case where the fracture grows from a state where themolten processed region forming the cutting start region is molten and acase where the fracture grows when the molten processed region formingthe cutting start region is re-solidified from the molten state. Ineither case, the molten processed region is formed only within thesilicon wafer, and thus is present only within the cross section aftercutting as shown in FIG. 12. When a cutting start region is thus formedwithin the object by a molten processed region, unnecessary fracturesdeviating from a cutting start region line are harder to occur at thetime of cleaving, whereby cleavage control becomes easier.

(3) Case where the Modified Region is a Refractive Index Change Region

An object to be processed (e.g., glass) is irradiated with laser lightwhile locating a light-converging point within the object under acondition with a field intensity of at least 1×10⁸ (W/cm²) at thelight-converging point and a pulse width of 1 ns or less. Whenmultiphoton absorption is generated within the object with a very shortpulse width, the energy caused by multiphoton absorption is notconverted into thermal energy, whereby an eternal structure change suchas ion valence change, crystallization, or orientation polarization isinduced within the object, thus forming a refractive index changeregion. The upper limit of field intensity is 1×10¹² (W/cm²), forexample. The pulse width is preferably 1 ns or less, for example, morepreferably 1 μs or less. The forming of a refractive index change regionby multiphoton absorption is disclosed, for example, in “Forming ofPhotoinduced Structure within Glass by Femtosecond Laser Irradiation”,Proceedings of the 42nd Laser Materials Processing Conference (November1997), pp. 105-111.

While the cases (1) to (3) are explained in the foregoing as a modifiedregion formed by multiphoton absorption, a cutting start region may beformed as follows while taking account of the crystal structure of awafer-like object to be processed, its cleavage characteristic, and soforth, whereby the object can be cut with a high precision by a smallerforce from the cutting start region acting as a start point.

Namely, in the case of a substrate made of a monocrystal semiconductorhaving a diamond structure such as silicon, it will be preferred if acutting start region is formed in a direction extending along a (111)plane (first cleavage plane) or a (110) plane (second cleavage plane).In the case of a substrate made of a III-V family compound semiconductorof sphalerite structure such as GaAs, it will be preferred if a cuttingstart region is formed in a direction extending along a (110) plane. Inthe case of a substrate having a crystal structure of hexagonal systemsuch as sapphire (Al₂O₃), it will be preferred if a cutting start regionis formed in a direction extending along a (1120) plane (A plane) or a(1100) plane (M plane) while using a (0001) plane (C plane) as aprincipal plane.

When the substrate is formed with an orientation flat in a direction tobe formed with the above-mentioned cutting start region (e.g., adirection extending along a (111) plane in a monocrystal siliconsubstrate) or a direction orthogonal to the direction to be formed withthe cutting start region, the cutting start region extending in thedirection to be formed therewith can be formed easily and accuratelywith reference to the orientation flat.

The preferred embodiment of the present invention will now be explained.FIG. 14 is a plan view of an object to be processed in the laserprocessing method in accordance with this embodiment, whereas FIG. 15 isa partial sectional view of the object taken along the line XV-XV ofFIG. 14.

As shown in FIGS. 14 and 15, the object 1 in this embodiment comprises asubstrate 4 having a thickness of 300 μm made of silicon, for example,and a laminate part 16, formed on the front face 3 of the substrate 4,including a plurality of functional devices 15. Each functional device15 includes an interlayer insulating film 17 a laminated on the frontface 3 of the substrate 4, a wiring layer 19 a disposed on theinterlayer insulating film 17 a, an interlayer insulating film 17 blaminated on the interlayer insulating film 17 a so as to cover thewiring layer 19 a, and a wiring layer 19 b disposed on the interlayerinsulating film 17 b. The wiring layer 19 a and the substrate 4 areelectrically connected to each other by a conductive plug 20 apenetrating through the interlayer insulating film 17 a, whereas thewiring layers 19 a and 19 b are electrically connected to each other bya conductive plug 20 b penetrating through the interlayer insulatingfilm 17 h.

Examples of the functional device 15 include semiconductor operatinglayers formed by crystal growth, light-receiving devices such asphotodiodes, light-emitting devices such as laser diodes, circuitdevices formed as circuits, and semiconductor devices.

While a number of functional devices 15 are formed like a matrix indirections parallel and perpendicular to an orientation flat 6 in thesubstrate 4, for example, the interlayer insulating films 17 a, 17 b arealso formed between adjacent functional devices 15, 15 so as to coverthe whole front face 3 of the substrate 4.

Thus constructed object 1 is cut into the individual functional devices15 as follows. First, as shown in FIG. 16( a), a protective tape 22 isattached to the object 1 so as to cover the laminate part 16.Subsequently, as shown in FIG. 16( b), the object 1 is secured onto amounting table (not depicted) of the laser processing apparatus suchthat the rear face 21 of the substrate 4 faces up. Here, the protectivetape 22 keeps the laminate part 16 from coming into direct contact withthe mounting table, whereby each functional device 15 can be protected.

Then, lines to cut 5 are set like a lattice (see broken lines in FIG.14) so as to pass between adjacent functional devices 15, 15. Thesubstrate 4 is irradiated with laser light L while using the rear face21 as a laser light entrance face and locating a light-converging pointP within the substrate 4 under a condition where multiphoton absorptionoccurs, whereas the mounting table is moved such that the lines to cut 5are scanned with the light-converging point P.

While each line to cut 5 is scanned six times with the light-convergingpoint P, the distance from the rear face 21 to the light-convergingpoint P is changed such that one row of quality modified region 71,three rows of divided modified regions 72, and two rows of HC (half cut)modified regions 73, which are successively arranged from the front face3 side, are formed row by row within the substrate 4 along each line tocut 5. Each of the modified regions 71, 72, 73 becomes a cutting startregion when cutting the object 1. Since the substrate 4 in thisembodiment is a semiconductor substrate made of silicon, each of themodified regions 71, 72, 73 is a molten processed region. As with theabove-mentioned modified region 7, each of the modified regions 71, 72,73 may be constituted by a continuously formed modified region ormodified regions formed intermittently at predetermined intervals.

When the modified regions 71, 72, 73 are formed row by row successivelyfrom the side farther from the rear face 21 of the substrate 4, nomodified region exists between the rear face 21 acting as the laserlight entrance face and the light-converging point P of laser light,whereby the modified regions formed beforehand neither scatter norabsorb the laser light L at the time of foiling the modified regions 71,72, 73, for example. Therefore, the modified regions 71, 72, 73 can beformed accurately within the substrate 4 along the lines to cut 5. Whenthe rear face 21 of the substrate 4 is used as the laser light entranceface, the modified regions 71, 72, 73 can be formed accurately withinthe substrate 4 along the lines to cut 5 even if a member (e.g., TEG)reflecting the laser light L exists on the lines to cut 5 in thelaminate part 16.

After the modified regions 71, 72, 73 are formed, an expandable tape 23is attached to the rear face 21 of the substrate 4 in the object 1 asshown in FIG. 17( a). Subsequently, the protective tape 22 is irradiatedwith UV rays as shown in FIG. 17( b), so as to lower its adhesive force,and the protective tape 22 is peeled off from the laminate part 16 ofthe object 1 as shown in FIG. 18( a).

After the protective tape 22 is peeled off, the expandable tape 23 isexpanded as shown in FIG. 18( b), so as to generate fractures from themodified regions 71, 72, 73 acting as start points, thereby cutting thesubstrate 4 and laminate part 16 along the lines to cut 5 and separatingthus cut semiconductor chips 25 (semiconductor apparatus) from eachother.

A method of forming the modified regions 71, 72, 73 will now beexplained in detail. FIG. 19 is a plan view showing a part of the object1 formed with the modified regions 71, 72, 73, whereas FIG. 20 is apartial sectional view of the object 1 taken along the line XX-XX ofFIG. 19.

The quality modified region 71 is formed by selectively switchingbetween continuous oscillation and pulsed oscillation at the time ofirradiation with the laser light L. The oscillation of laser light L canbe changed by a power controller (not depicted) for regulating the laserlight L, for example. Oscillating the laser light L pulsewise canreliably form the quality modified region 71 within the substrate 4,since the energy is higher than in the case continuously oscillating thelaser light L and goes over a threshold for processing. Therefore,oscillating the laser light pulsewise along desirable parts RP in thelines to cut 5 and continuously along parts (predetermined parts) RCother than the desirable parts RP can reliably form the quality modifiedregion 71 within the substrate 4 along the desirable parts RP.

Continuously oscillating the laser light L along the predetermined partsRC can also reduce damages imparted to the laminate part 16 by the laserlight L as compared with the case oscillating the laser light Lpulsewise along the predetermined parts RC, since the energy of thelaser light L oscillated continuously is low and does not go over athreshold for processing. Therefore, the accuracy in cutting thelaminate part 16 in the predetermined parts RC in the lines to cut 5 canbe improved when cutting the substrate 4 and laminate part 16. Hence, asshown in FIG. 18( b), the cut section (side face) 4 a of the substrate 4and the cut section (side face) 16 a of the laminate 16 become highlyaccurate cut sections with suppressed irregularities in thesemiconductor chips 25 manufactured by using the laser processing methodof this embodiment.

In this embodiment, as shown in FIGS. 19 and 20, a metal film M isprovided within the laminate part 16 along the predetermined parts RC inthe lines to cut 5. From the viewpoint of reducing damages to theabove-mentioned laminate part 16, it will be preferred if no qualitymodified region 71 is formed within the substrate 4 along thepredetermined parts RC. When the metal film M is provided within thelaminate part 16, the laminate part 16 is more likely to be damaged. Thefollowing seem to be reasons why such damages occur. Under the influenceof aberrations and the like of lenses converging the laser light L, apart of components of the laser light L may be converged by the metalfilm M. In this case, the laser light L is reflected by the metal filmM, so that thus reflected light may faun modified regions within thesubstrate 4 and laminate part 16 or at the interface between thesubstrate 4 and laminate part 16. In the case where the rear face 21 ofthe substrate 4 is an entrance face in particular, aberrations of lensesbecome more influential when forming modified regions on the sidefarther from the entrance face. Also, modified regions are likely to beformed within the laminate part 16, since the threshold required forforming modified regions is lower in the laminate part 16 than in thesubstrate 4. However, causes of damages to the laminate part 16 are notrestricted to those mentioned above.

Examples of the metal film M include metal wiring and metal padsconstituting a test element group (TEG). The metal film M may be a filmwhich peels off upon heating. In place of the metal film M, aninsulating film such as low dielectric constant film (low-k film) may beprovided within the laminate part 16. The insulating film may be a filmwhich peels off upon heating. An example of the low dielectric constantfilm is a film having a dielectric constant lower than 3.8 (thedielectric constant of SiO₂).

Preferably, as shown in FIG. 19, the laser light L is oscillatedpulsewise in parts CP where the lines to cut 5 intersect. This canreliably form the quality modified region 71 within the substrate 4along the parts CP where the lines to cut 5 intersect. As a consequence,when cutting the substrate 4 and laminate part 16, the lines to cut 5can prevent chipping and the like from occurring in the parts CP wherethe lines to cut 5 intersect. Therefore, the accuracy in cutting thesubstrate 4 and laminate part 16 can further be improved.

Preferably, as shown in FIG. 20, one row of quality modified region 71is formed at a position where the distance between the front face 3 ofthe substrate 4 and the front-side end part 71 a of the quality modifiedregion 71 is 5 μm to 20 μm, or at a position where the distance betweenthe front face 3 of the substrate 4 and the rear-side end part 71 b ofthe quality modified region 71 is [5+(the thickness of the substrate4)×0.1] μm to [20+(the thickness of the substrate 4)×0.1] μm. Here, ifan expandable tape 23 as an expandable film is attached to the rear face21 of the substrate 4 and expanded, for example, the substrate 4 andlaminate part 16 will be cut along the lines to cut 5. If the modifiedregion 71 is formed at any of the above-mentioned positions, thelaminate part 16 (constituted by the interlayer insulating films 17 a,17 b here) can be cut with a high precision. Even when the substrate 4has a large thickness, e.g., 300 μm, the substrate 4 and laminate part16 can be cut with a high precision.

In the forming of the divided modified regions 72, three rows, forexample, of modified regions 72 are formed as a series in the thicknessdirection of the substrate 4. Further, in the forming of the HC modifiedregions 73, two rows, for example, of HC modified regions 73 are formed,so as to generate fractures 24 extending along the lines to cut 5 fromthe HC modified regions 73 to the rear face 21 of the substrate 4.Depending on forming conditions, the fracture 24 may occur between thedivided modified region 72 and HC modified region 73 adjacent to eachother. When the expandable tape 23 is attached to the rear face 21 ofthe substrate 4 and expanded, fractures proceed smoothly from thesubstrate 4 to the laminate part 16 by way of the divided modifiedregions 72 formed by three rows as series in the thickness direction,whereby the substrate 4 and laminate part 16 can be cut accurately alongthe lines to cut 5.

The divided modified regions 72 are not restricted to three rows as longas they can smoothly advance fractures from the substrate 4 to thelaminate part 16. In general, the number of rows of divided modifiedregions 72 is reduced as the substrate 4 becomes thinner, and isincreased as the substrate 4 becomes thicker. The divided modifiedregions 72 may be separated from each other as long as they can smoothlyadvance fractures from the substrate 4 to the laminate part 16. The HCmodified region 73 may be one row as long as the fractures 24 canreliably be generated from the HC modified region 73 to the rear face 21of the substrate 4.

Though a preferred embodiment of the present invention is explained indetail in the foregoing, the present invention is not limited to theabove-mentioned embodiment.

For example, though the above-mentioned embodiment selectively switchesbetween pulsed oscillation and continuous oscillation when forming thequality modified regions 71, the pulsed oscillation and continuousoscillation may selectively be switched therebetween when forming othermodified regions. Examples of the other modified regions include dividedmodified regions 72 and HC modified regions 73. Among them, from theviewpoint of improving the accuracy in cutting, it will be preferred ifthe pulsed oscillation and continuous oscillation may selectively beswitched therebetween when forming the quality modified region 71positioned closest to the device.

The object 1 may be a GaAs wafer or a silicon wafer having a thicknessof 100 μm or less. In these cases, foaming one row of modified regionwithin the object 1 along the lines to cut 5 can cut the object 1 with asufficiently high precision.

The modified regions 71, 72, 73 are not limited to those formed by themultiphoton absorption generated within the object 1. The modifiedregions 71, 72, 73 may also be formed by generating optical absorptionequivalent to the multiphoton absorption within the object 1.

Though a semiconductor wafer made of silicon is used as the object 1 inthis embodiment, the material of the semiconductor wafer is not limitedthereto. Examples of the semiconductor wafer material include group IVelement semiconductors other than silicon, compound semiconductorscontaining group IV elements such as SiC, compound semiconductorscontaining group III-V elements, compound semiconductors containinggroup II-VI elements, and semiconductors doped with various dopants(impurities).

An example of the laser processing method in accordance with theembodiment will now be explained in detail, though the present inventionis not limited to this example. FIGS. 21( a) and 21(b) are plan views ofan object to be processed in the laser processing method in accordancewith this example. FIG. 22 is a view showing a photograph of the cutsection 4 a of the substrate in the object shown in FIG. 21( b), whilecorresponding to FIG. 20.

First, the laser light L is oscillated pulsewise along desirable partsRP in a line to cut 5 positioned between functional devices 15, 15, soas to form a quality modified region 71 within the object. On the otherhand, the laser light L is continuously oscillated along a predeterminedpart RC in the lines to cut 5, whereby no quality modified region 71 isformed within the object. Here, the rear face 21 of the substrate is thelaser light entrance face. Next, divided modified regions 72 and HCmodified regions 73 are formed along the line to cut 5. As a result,though a metal film M is included in the laminate part 16 extendingalong the predetermined part RC in the line to cut 5, no damages to thelaminate part 16 caused by laser light are seen as shown in FIG. 21( a).

After the modified regions 71, 72, 73 are formed, an expandable tape isattached to the object and is expanded by an expander, so as to cut theobject (see FIG. 21( b)). As shown in FIG. 21( b), the cut section 16 ais seen to have been cut with a high precision without irregularities. Aphotograph taking the cut section 4 a of the substrate in thus cutobject is shown as a drawing in FIG. 22. As shown in FIG. 22, no qualitymodified region is formed within the object along the predetermined partRC in the line to cut 5.

Laser processing conditions for forming the modified regions 71, 72, 73will now be explained. The pulse width of laser light L is 180 ns. Theinterval between irradiating positions of laser light L (pulse pitch) is4 μm. The frequency of laser light L is 75 kHz. The relationship amongthe distance from the rear face 21 to become the entrance face 21 to thelight-converging point P (light-converging point position), the energyof laser light L, and the unit time energy is as shown in Table 1.

TABLE 1 Light-converging point Energy Unit time Energy position (μm)(μJ) (W) Quality modified 290 9.5 0.71 region 71 Divided modified 180 151.13 region 72 Divided modified 144 15 1.13 region 72 HC modified 85 50.38 region 73 HC modified 46 5 0.38 region 73

On the other hand, FIGS. 23( a) and 23(b) are plan views of the objectin an example of laser processing method. FIG. 24 is a view showing aphotograph of a cut section 104 a of the substrate in the object shownin FIG. 23( b).

First, a line to cut 105 positioned between functional devices 115, 115is irradiated with laser light, so as to form modified regions 171, 172,173 within the object. Here, the rear face 121 of the substrate is thelaser light entrance face. Since the line to cut 105 is irradiated withthe laser light, damages such as peeling of the film occur in a laminatepart 116 containing a metal film 100M in this case as shown in FIG. 23(a).

After the modified regions 171, 172, 173 are formed, an expandable tapeis attached to the object and is expanded by an expander, so as to cutthe object (see FIG. 23( b)). As shown in FIG. 23( b), irregularitiesare seen in a cut section 116 a, thus verifying that the accuracy incutting is insufficient. A photograph taking the cut section 104 a ofthe substrate in thus cut object is shown as a drawing in FIG. 24. Asshown in FIG. 24, a modified region 171 is formed within the objectalong the line to cut 105.

INDUSTRIAL APPLICABILITY

The present invention can reliably form a modified region within anobject to be processed along a desirable part in a line to cut.

1-8. (canceled)
 9. A semiconductor apparatus, comprising: a substratehaving a front face, a rear face, and a side wall; a laminate formed onthe front face of the substrate, the laminate having a side wall, theside wall of the laminate being flush with the side wall of thesubstrate; a metal film embedded in the laminate, the metal filmextending substantially in parallel to the front face of the substrateand a side of the metal film being exposed at the side wall of thelaminate; and a modified region having a longitudinal shape extendingsubstantially in parallel to the front face of the substrate andarranged along the side wall of the substrate, the modified region notbeing formed at an area in the substrate, wherein the metal film isarranged above the area in the substrate where the modified region isnot formed.
 10. The semiconductor apparatus according to claim 9,wherein the semiconductor apparatus has a rectangular shape, and thesubstrate and the laminate each have four side walls that are flush withrespect to each other, and wherein the modified region is located oneach of the sidewalls of the substrate.
 11. The semiconductor apparatusaccording to claim 10, wherein the area is located at a central area ofthe side wall of the substrate, and the modified region is firmed at theedges of the side wall of the substrate.
 12. The semiconductor apparatusaccording to claim 9, wherein the modified region is at least one of amolten processed region of the substrate, a phase change region of thesubstrate, and a region having a changed crystal structure.
 13. Thesemiconductor apparatus according to claim 9, wherein the modifiedregion is a region in the substrate that has changed from a firststructure into a second structure, the first structure being at leastone of a monocrystalline structure, an amorphous structure, and apolycrystalline structure, the second structure being at least one of amonocrystalline structure, an amorphous structure, and a polycrystallinestructure, and the first and the second structures being different fromeach other.
 14. The semiconductor apparatus according to claim 10,further comprising: a functional device located in the laminate but notwithin a rectangular border area that is arranged along the side wallsof the laminate, wherein the metal film is located in the rectangularborder area.